Adding insulation to crawl space

Q. We have a crawl space underneath one room and it has batt insulation and only chicken wire covering it. One question is can this insulation get mildewed? Sometimes we can smell an odor in this room but not sure if it is coming from underneath. Not sure how long this insulation has been there but it is fairly dirty and I assume that some moisture would be present in it.

Also, along with replacing it with more batt insulation, can a rigid board cover the new one for better protection? Thank you.

A. Is the crawlspace floor covered with 6-mil plastic to contain the soil’s moisture? If not, it should be. The perimeter of the plastic at the foundation should be weighted down with either a bit of the soil, a few brick batts, or pieces of pressure-treated wood scraps. All seams should be generously overlapped by at least two feet.

If there is some leakage from outside, the plastic should be brought up the foundation walls to a line above the outside grade and taped or otherwise fastened to clean walls. The plastic must be laid so that water leaking is directed below all seams in the plastic. For example, any plastic laid in the center, over the plastic around the perimeter, must be laid on top of the perimeter pieces.

If the existing fiberglass batts are dirty, it indicates that they may have been subjected to air movements. Do you have crawlspace vents in the foundation, and are they open permanently? Once the soil is thoroughly covered with plastic, you should close or seal these vents to keep air out. You can install pieces of rigid insulation in them.

From time to time, give the crawlspace the nose test; it should smell OK. If the fiberglass batts feel dry, there should be no need to replace them, as they are non-absorbent. Any moisture on the fibers would be there simply as a result of surface tension.

Once you have made the suggested corrections, there is no need to replace the chicken wire with a rigid board. Unless the rigid board material you would choose is moisture permeable, it could trap any living space moisture migrating through the floor system in the joist cavities and cause problems.